Tolerances

Generally speaking, sandcasting does not provide high-definition, mirror finishes, and close tolerances. For some applications, a foundry-finish and industry-accepted standards for size and shape tolerancing are acceptable for the product in question.

The natural tolerances of the sandcasting process appear differently with regard to the orientation of the surfaces under consideration to the parting line of our mold. Surfaces lying parallel to the parting line, but on the same side, exhibit better natural variability than if the surfaces are parallel, but on opposite sides of the Parting Line. Features and surfaces perpendicular to the parting line generally show a little more variability if they are on opposite sides — because of mold shift and core shift.

OrientationNatural Variability
Same side of P/Line+/- 0.005
Opposite sides, parallel to P/L+/- 0.010
Opposite sides, Perpendicular to P/Line+/- 0.015

These tolerances are discussed in detail at the website of the Aluminum Association, www.aluminum.org.

For many purposes a machined casting is required. When the foundry engineer knows that an application requires close tolerancing, then features are added to the casting design — to enhance the final quality of the machined casting. Critical surfaces may be cast with added material, so that the machining step will remove surface irregularities. The casting may be heat-treated, to boost tensile strength and provide for uniform chip-formation during machining.

Location of gating and risers is shared in consultation with the machinist, so that areas of the part subject to added variation may be avoided in locating and clamping the part during the beginning of the machining process.

In this way, much value can be derived from employing a machined casting. By allowing the casting and machine processes to supplement one another, Machine process time can be held to a minimum, tooling costs can be held low, yet the resultant part can be of high quality, at an acceptable cost level.